IS 14245: Protective atmosphere for heat treatment furnaces - Recommendations

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IS 14245 (1995) : Protective atmosphere for heat treatment
furnaces - Recommendations [MTD 26: Industrical Fuel Fired
Furnaces]

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PROTECTED BY COPYRIGHT

1814245:1995

Indian Standard

PROTECTIVE ATMOSPHERE FOR HEAT

TREATMENT FURNACES —

RECOMMENDATIONS

UDC 66.041.2

©BIS 1995

BUREAU OF INDIAN STANDARDS

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI 110002

March 1995 Price Group 3

Industriai Fuel Fired Furnaces Sectional Committee, MTD 26

FOREWORD

This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the
Industrial Fuel Fired Furnaces Sectional Committee had been approved by the Metallurgical Engineering
Division Council.

Although heating of solids in an industrial furnace is intended to be basically a physical process, chemical
reaction between the charge and the surrounding medium, i.e. the furnace atmosphere cannot be avoided.
Such chemical reactions not only vary with the operating temperature but also with the composition of
the charged material and the atmosphere that is prevalent inside the furnace. Therefore, controlling of
the gaseous medium inside the working chamber of a furnace is essentially required for both heating and
heat treatment processes.

Based on the initial composition and application requirement of a metal upon heat treatment, different
metallurgical properties are desired in various metals and alloys and different heat treatment processes
are carried out. To satisfy such multifarious heat treatment requirements, types, composition/properties
as well as application possibilities of protective atmospheres also vary widely.

In view of the above, it is extrem.ely difficult to set-forth any specific or well-defined standards or
prescriptions for the use of different types of protective atmospheres for various heat treatment opera-
tions. In this standard, an effort is, however, made to stipulate general guidelines with respect to the use
of extraneously prepared protective atmosphere having definite composition to be introduced inside the
furnace working chamber during heat treatment.

For the purpose of deciding whether a particular requirement of this standard is complied with, the final
value, observed or calculated, expressing the result of a test or analysis, shall be rounded off in accordance
with IS 2 : 1960 'Rules of rounding off numerical values (revised)\ The number of significant places
retained in the rounded off value should be the same as that of the specified value in this standard.

IS 14245 : 1995

Indian Standard

PROTECTIVE ATMOSPHERE FOR HEAT

TREATMENT FURNACES —

RECOMMENDATIONS

1 SCOPE

This standard covers the recommendations for
protective atmosphere for heat treatment furnaces.

2 FUNCTIONS

2.1 Properly applied and controlled furnace
atmosphere serves one or more of the following
functions.

2.1.1 Protective environment to guard against
adverse effects on metals during the processing
work or storage.

2.1.2 Surface cleansing of parts being treated in
some processes or prevent stainingdue to oxidation
of rolling oils/drying compounds.

2.1.3 Addition or removal of elements to achieve
particularproperty of metal through chemical reac-
tions during some of the heat treatment processes.

2.1.4 Certain protective environments may be
used to increase heat transfer efficiency.

2.2 Oxygen is the most reactive constituent in air,
which at elevated temperatures, reacts with most
metals to form oxides or scales. In addition, oxygen
reacts with carbon that is dissolved in steel to lower
the carbon content of its surface.

2.3 For heat treatment of materials, normally
electrically heated or indirect fuel fired furnaces are
recommended. This is primarily because both
oxygen in the air inside the furnace as well as
oxygen, water and carbon dioxide in the products of
combustion from fuel firing inside the furnace
chamber could deteriorate the metal surface.
Therefore, air containing Dxygcn should be driven
out from the furnace chamber for maintaining the
so called 'neutral atmosphere' or to maintain par-
ticular composition in the furnace atmosphere in
order to achieve the desired reaction between the
charged metal and the atmosphere inside the heat
treatment chamber.

3 SELECTION

Selection of the type and correct composition of
protective atmosphere should be based on:
— Material and composition of charge to be
heat-treated.

— Ultimate desired property of the heat
treated products.

4 CLASSIFICATION AND SOURCEy
PREPARATION

4.1 Commercially important protective atmos-
pheres are classified on the basis of the method of
preparation of the original constituents employed
and are commonly referred to in the industry by
their generic name or sometimes by manufacturer's
trade names. There are a few classified groups,
broadly designated and defined as under, of protec-
tive atmospheres most of which are extraneously
prepared in specially designed atmosphere
generators either by Exothermic or Endothermic
reactions.

4.1.1 Exothermic Base

Formed by partially or complete combustion of
hydro-carbon with closely controlled air/fuel ratio.
Under the above category falls prepared nitrogen
base atmospheres.

4.1.2 Prepared Nitrogen Base

Atmosphere formed by combustion of mixture of
air and fuel gas from which almost all of the CO2
and H2O vapour have been removed.

4.1.3 Endothermic Base

Formed by catalytic reaction of a mbcture of
hydrocarbon fuel gas and air in an externally heated
catalyst filled chamber and subsequent cooling.

4.1.4 Ammonia Base Atmosphere

Tliese are raw-ammonia, dissociated ammonia or
partially or completely combusted dissociated
ammonia with subsequent cooling and drying.

4.1.5 ExothermiC'Endothermic Base

Formed by complete combustion of a mixture of
fuel gas and air, removing H2O vapour and re-form-
ing CO2 into CO by means of reaction with fuel gas
in a externally heated catalyst filled chamber.

4.2 Besides the above, following other mediums
can also be recommended for use as protective
atmosphere for various heat treatment operations.

1

IS 14245 : 1995

4.2.1 Commercial Nitrogen Base Atmosphere

In this the major atmosphere component is in-
dustrial gas nitrogen which is supplied to the fur-
nace from a pressure and flow rate. Nitrogen serves
as a pure, dry, inert gas that provides an efficient
purging and blanketing function within the heat
treating furnace. The nitrogen stream is often en-
riched with a reactive component and the resulting
composition and flow rate are determined by the
specific furnace design, temperature and materials
being heat treated.

4.2.2 Charcoal Base Atmosphere

4.2.3 Steam

4.2.4 Dry Hydrogen

Commercially available hydrogen, which is 98 to
99.9 percent pure, can be used for treatment of
various metals and alloys in its dry form.

4.2.5 Inert Gases

5 TYPICAL COMPOSITION

Typical compositions of various protective
atmospheres are given in Table 1.

6 COMMON USE

Formed by passing air through a bed of Common uses of various portective atmospheres
incandescent charcoal. are given in Table 2.

Table 1 Typical Compositions of Protective Atmospheres

SI No.

(1)

Type of AiiROsphere

(2;

1 Exothermic Base

i) Exothermic composition +
cooling

ii) Exothermic combustion +
cooiing+diying (removal
of HiO vapour)

2 Prepared Nitrogen Base

Ngr;sr8! CompGsitBf

BR (Volume Percen!)

Dew Point, "C

^Hi CO

C02

N2

CH4'*

(3) (4)

(5)

(6)

(7) (8)

1-14 Ml

11-58

7-70

— 5° above cooHng
water temperature

1-14 1-11

11-58

7-70

— -40 to -30

i)

Exothermic combustion +
CO2 removal + drying

0.5-14

0.5-11

0.001-0.02

99.75

—

-70 to -50

M)

Exothermic combustion +
single stage CO reforming
and CO2 removal +drying

1-22

0.1-3

0.001-0.02

99.75

"

-70 to -50

iii)

Exothermic combustion +
two stage CO reforming
and CO2 removal + drying

1-25

<0,1

<0.02

99.75

—

-50

3 Endothermic Base

Partial endothermic
reaction and indirect
heating with catalyst +cooling

28-38

20-24

0-0,5

52-38

— +5 to -10

4 Ammonia Base

Partially or completely
combusted dissociated
ammonia +cooling+ drying

75

25

-70

5 ExothermiC'Endothermic Base

6 Charcoal Base

20-21 17-19 — 60-63 —

1.5-7 30-32 30-32 1-2 Trace

Balance Balance upto 0.5

NOTE — Traces of other harmful constituents like O2, SO2, H2S, etc, shall be maintained within tolerance limits mutually
agreed between the supplier and user.

IS 14245 : 1995

Table 2 Common Uses of Protective Atmospheres

( Clause 6 )

SI No.

Type of Atmosphere

(1)

(2)

Lean exothermic

ii)

Rich exothermic

iii)

Lean prepared N2

iv)

Rich prepared N2

V)

Lean endothermic

vi)

Rich endothermic

vii)

Dissociated ammonia

viii)

Lean combusted ammonia

ix)

Rich combusted ammonia

X)

Lean exothermic-endothermic

xi)

Charcoal

xii)

Steam

xiii)

DryH2

xiv)

laerl gases

7 TYPE/DESIGN OF GENERATORS

There are two basic categories of generators viz.
7.1 Exothermic Gas Generator

Basic design should include:

— Air blower with inlet filter

— Fuel and air metering equipment

— Combustion chamber

— Burner

— Cooler

— Necessary purification units

— Control panel

— Safety equipments

Design may vary and choice or suitablity of design
generally influenced by application considerations.
The burners may also vary in construction. Those
offering easy and rapid lighting up so that the gen-
erator can go into production within a few minutes
of start-up are prcl'errcd.

Coolers can be:

— Direct water/gas contact type.

— Indirect shell/tube type, which will be used
where low O2 content of exo-gas is vital and
conservation of water through recirculation
is desired.

7.2 luidothermic Gas Generator

Basic design should include:

— Air inlet filter

Common Use

(3)
Oxide coating of steel

Diy annealing, copper brazing, sintering, nonferrous annealing
Neutral heating

Anneahng, brazing stainless steel
Skin hardening/carbon retoration
Gas carburising

Brazing, sintering, bright annealing
Neutral healing

Sintering stainless steel powders
Clean hardening

Carburizing, hardening, annealing of high carbon steels with
scahng or surface decarburisation

Scale free tempering and str .;ss relieving of ferrous metals in
the temperature range of 350-650°C
Annealing of stainless steel, low carbon steel electric grade
steel, metal powders

Sintering of refractory materials, metal powder compacts
Brazing of stainless steel heat resistant steel, copper, etc
Thermal processing of reactive metals and their alloys

— Fuel and air metering and mixing equip-
ment/device

— One or more heat resisting alloy retorts for
catalystic cracking

— Insulated heating chamber

— Heating system

— Cooler

— Control Panel

Process and heating being completely separate, fuel
gas, fuel oil or electricity may be used as heating
medium.

8 CONTROL EQUIPMENT/ACCESSORIES

Control parameters should be based on desired
composition and properties of atmosphere gas and
the design of the generator. Control elements com-
monly include:

— Air filter and blower

— Air flow metering

— Fuel gas pressure regulating valve

— Fuel gas flow metering

— Solenoid safety shut-off valve

— Fuel gas/air ratio regulator

— Temperature control equipments/
instruments

— Protective atmosphere flow metering

— Control Panel for mounting of control
equipments, accessories for safety control,
standard electrical switch-gears

IS 14245 : 1995

9 HAZARDS AND SAFETY PRECAUTIONS

Safety hazards fall into three groups.

9.1 Fire

Atmospheres containing more than 4% combus-
tibles are classified as flammable. Combustibles
like H2, CO and CH4 and other hydro-carbon fuel
gases should never be admitted to a furnace cham-
ber at temperature below 760°C.

9.2 Explosion

Mixtures of air and combustible gases will explode
when ignited if the temperature of combustion
chamber is at or "below 76CrC. An adjacent cold
chamber of vestibule can then be flared as the at-
mosphere flows from the furnace to the vestibule
until it is free of oxygen from air. The vestibule can
then be closed. The positive flow of atmosphere
through the furnace and adjoining vestibule can
then be burned. An ignited effluent from an atmos-
phere furnace is an immediate visual sign that a safe
condition prevails. Any sudden stoppage of flame
indicates an alarm situation and suitable inert gas
purging should start immediately.

9.3 Toxicity

Many atmosphere gases are toxic. Burning them at
furnace exits reduces their chemistry to the produc-
tion of combustion which then should be vented
outside the building to prevent dilution of the avail-
able oxygen within the building. Some protective
atmosphere may require treatment before they are
vented out. For example, in nitriding furnaces, it is
necessary to pass the partial dissociated NH3 gas
through water so that the excess NH3 is dissolved
in water and the surrounding work space is not
contaminated. Whenever handling or combustible
or explosive gas proper routine checking is required
to ensure that the pipeline and the equipment is
free from leakage. Ventilation of the building con-
taining atmosphere generators and controlled at-
mosphere heat treating furnaces is a major safety
consideration.

10 PRECAUTIONS

10.1 It is essential to follow safe starting and stop-
ping procedure for any equipment using combi^-
tible/explosive gas. Property illustrated
instructions should be given; by the manufacturer
of furnace and gas generator. However, the follow-
ing general procedure can be followed.

10.1.1 When using rich, combustible gases like,
Endo, Rich Exo, cracked NH3, H2, etc, gas should
not be inserted in the furnace below 16(fC
temperature.

10.1.2 In the applications when a protective gas is
required even when the charge is at lower range of
temperature, suitable inert gas (generally N2)
should be first introduced and then it can be
replaced by the rich gas.

10.1.3 In the event of normal shut-down or any
sudden stoppage due to equipment failure, power
failure, etc. Inert gas purging should be started as
already described at 9.2.

10.1.4 Statutory requirements of appropriate
authoriti^ should be followed strictly.

11 MAINTENANCE REQUIREMENTS

11.1 Properly documented and illustrated opera-
tion and maintenance instructions should be fur-
nished by the supplier of gas generators.

11.2 Following schedule may, however, be
suggested as normal maintenance requirements
for:

11.2.1 Endothermk Generators

11.2.1.1 Weekfy and/or monthly

— Burn out carbon in generator.

— Clean air filter.

— Check calibration of control instruments.

— Inspect thermocouples.

11.2.1.2 Annual, or as prescribed in operating
instructions

— Test all safety controls.

— Inspect catalyst; in retort and fill to
appropriate level or replace.

— Inspect and clean burners.

— Check compressor blades and bearings in
mixer pump and lubricate if necessary.

— Check motor bearings; on mfacer pump.

— Clean gas lines to furnaces, if necessary.

11.2.2 Prepared Nitrogen Base Generator

11.2.2.1 Twice per 8 hour shift

— Observe and record variables such as
temperatures, pressures, flow rates and
liquid levels.

— Observe and record details of abnormalities
such as improper function of traps, safety
switches and excessive pump noise or
leakage. Take immediate corrective action.

— Drain designated non-automatic traps.

11.2.2.2 Monthly or as prescribed in operating
procedures

— Clean or replace all filters/strainers as
required.

1814245:1995

— Lubricate all plug-type valves using — Check primaiy gas analysis and make adjust-
appropriate lubricant. ment as necessaiy.

— Oieck operation of fuel safety-shut valve — Check operation and linkage of
and pressure switches. temperature control valve.

— Oean all traps unless safety considerations — Lubricate as required,
override. — Check drive belts.

— Check flame detector and clean as required.

U2S3 Monthly
11.23 Exo^hermiC'Endothennic Generator

11J3.1 Daihf ~ Cl»^k for leaks.

^^L MJauy _ Qj^i^ operation of combustion and fuel

— Check flow of cooling water. safeguard equipment such as manual reset

— Check dew point and secondary g^ analysis shut-off valves, pressure switches and fire-
and make adjustments as necessary. check.

— Check instruments. — Check and clean air filter as required.

— Check atmosphere cooler and cooling

IIJJJ Weekly distribution.

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This Indian Standard has been developed from Doc : No. MTD 26 ( 3468 ).

Amendments Issued Since Publication

Amend No.

Date of Issue

Text Affected

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